In the prior art, thin ceramic layers or coatings have been bonded to, or coated/deposited on metal substrates for support of the ceramic and as a means for dissipating heat generated by circuit components mounted thereon. However, such prior art arrangements have not provided the desired results. Traditionally, alumina (the hexagonal structure of Al.sub.2 O.sub.3) has been used with such metals as copper, aluminum, etc. because of the extensive knowledge and data available regarding these materials. Prior art devices using these materials have proven undesirable because of poor bond development between the alumina and the various metals used. In addition these materials in combination have been found to be incompatible with elevated temperature operations and are plagued by low dielectric properties and debonding when used at high power levels. More recently, efforts have been directed toward the use of ceramics thermal/plasma spray deposited on metal substrates. However, these efforts also have not been found to be satisfactory owing to inherent thermal and dielectric properties of the ceramics utilized. There appears to be a lack of readily available data and understanding concerning surface condition or treatment thereof to solve these long standing problems of the prior art. For example, there is disclosed in Sarin et al (U.S. pat. Ser. No. 4,406,668) and Komatsu et al (U.S. pat. Ser. No. 4,761,339) techniques for surface treatment of metals with ceramics for the purpose of providing ceramic coatings therefor that are resistant to oxidation at elevated temperatures, while Surisowa et al (U.S. pat. Ser. No. 4,539,251) is concerned with ceramic materials applied to the surface of metal tools used in cutting tools and other wear resistant tools which are subjected to elevated temperatures. None of these references address the problems of thermal conductivity, dielectric breakdown or other electrical characteristics of the ceramic in combination with the attendant metal substrates for purely electrical circuitry.
Other bonding techniques for electrical applications, i.e., the metallization of alumina and beryllia electronics substrates employ eutectic bonding of an oxidized copper foil surface to the ceramic at temperatures, e.g. 1065.degree. C., where the copper oxide is amorphous and bonds to the ceramic (See "Electronic Packaging Materials Science" Materials Research Society Symposia Proceedings, Vol. 40, Ed. E. A. Giess et al, p. 393, M. Wittmer, "Eutectic Bond of Copper to Ceramic"). Another prior art technique is to attach a metal heat sink to alumina by dip brazing or soldering using such materials as MoO.sub.3 -MnO or Ti-Cu (see M. Wittmer & Y. Kuromitsu, et al, ISHM 1990, p.19.).
In contrast to the prior art and in accordance with the techniques of the present invention, it has been discovered after extensive and costly investigation and experimentation that certain ceramics may be deposited as a layer on a metal support substrate by means of plasma spray with excellent bonding therebetween (bond-development) which may be further enhanced when the surface of such ceramic is laser-reflowed and then rapidly cooled for resolidification and recrystallization. As can readily be appreciated from the foregoing discussion of the prior art problems, it is desirable to find a method and ceramic materials by which a thin layer or coating of ceramic can readily be adapted for combination with metal substrates which will have the electrical and thermal properties, including good thermal conductivity, increased dielectric strength, and enhanced bond-development for high power and elevated temperature operation.